For exemplary purposes, the system for incorporating upper and lower counterweights of the present invention will be described in association with a scroll machine. It is to be understood that it is within the scope of the present invention to utilize the system of the present invention with any device which includes a brushless permanent magnet motor and which requires counterweights.
A class of machines exists in the art generally known as “scroll” machines for the displacement of various types of fluids. Such machines may be configured as an expander, a displacement engine, a pump, a compressor, etc., and many features of the present invention are applicable to any one of these machines. For purposes of illustration, however, the disclosed embodiments of the present invention are in the form of a hermetic refrigerant compressor powered by a brushless permanent magnet motor.
Generally speaking, a scroll machine comprises two spiral scroll wraps of similar configuration, each mounted on a separate end plate to define a scroll member. The two scroll members are interfitted together with one of the scroll wraps being rotationally displaced 180° from the other. The machine operates by orbiting one scroll member (the “orbiting scroll”) with respect to the other scroll member (the “fixed scroll” or “non-orbiting scroll”) to make moving line contacts between the flank surfaces of the wraps, thereby defining moving isolated crescent-shaped pockets of fluid. The spirals are commonly formed as involutes of a circle, and ideally there is no relative rotation between the scroll members during operation; i.e., there is purely curvilinear translation (i.e., no rotation of any line in the body). The fluid pockets carry the fluid to be handled from a first zone in the scroll machine where a fluid inlet is provided, to a second zone in the machine where a fluid outlet is provided. The volume of the sealed pockets change as they move from the first zone to the second zone. At any one instant in time, there will be at least one pair of sealed pockets; and when there are several pairs of sealed pockets at once, each pair will have different volumes. In a compressor, the second zone is at a higher pressure than the first zone and it is physically located centrally in the machine, the first zone being located at the outer periphery of the machine.
Two types of contacts define the fluid pockets formed between the scroll members. First, axially extending tangential line contacts are formed between the spiral faces of flanks of the scroll wraps. These tangential line contacts are caused by radial forces (flank sealing). Second, area contacts are formed between the plane edge surfaces (the “tips”) of each wrap and the opposite end plate (“tip sealing”). These area contacts are caused by axial forces. For high efficiency, good sealing must be achieved for both types of contacts.
The concept of a scroll-apparatus has, thus, been known for some time and has been recognized as having distinct advantages. For example, scroll machines have high isentropic and volumetric efficiency, and, hence, are relatively small and lightweight for a given capacity. They are quieter and more vibration-free than many compressors because they do not use large reciprocating parts (e.g., pistons, connecting rods, etc.) and because all fluid flows in one direction with simultaneous compression in plural opposed pockets, there are less pressure-created vibrations. Such machines also tend to have high reliability and durability because of the relatively few moving parts utilized, the relatively low velocity of movement between the scrolls, and an inherent forgiveness to fluid contamination.
The orbiting of one scroll member with respect to the other scroll member creates an imbalance which is typically counteracted using one or more counterweights. When a scroll machine is powered by an induction motor, the induction motor includes a rotor which is at least partially manufactured from cast aluminum. The cast aluminum portion of the rotor includes bars and end rings which allow for the prior art method used for attaching the counterweights. An upper counterweight is typically attached to the top end of the rotor to balance the scroll mass. A lower counterweight is typically attached to the lower end of the rotor to balance the moment mass.
On induction motors, the counterweights are typically attached to the rotor end rings by use of aluminum posts which are cast as part of the rotor. These cast posts extend through openings in the counterweights and the posts are swaged over to retain the counterweights on the rotor. Counterweights can also be pressed or bolted onto the rotating rotor or the drive shaft of the scroll machine. For reasons of low cost and minimal variation in the balancing mass, zinc die-cast counterweights have been the design of choice for comparable AC motor scroll compressors.
Brushless permanent magnet (BPM) motors have rotors that use permanent magnets which are unmagnetized and loose during assembly. BPM rotors typically do not include cast aluminum bars or end rings so counterweights cannot be swaged on as described above for the induction motor counterweights. The BPM rotor is made up of a stack of steel laminations with permanent magnets assembled in slots in the rotor stack. As noted above, the magnets are in an “unmagnetized” state during the assembly of the rotor. This structure is somewhat loose due to the fact that it does not have cast aluminum rotor bars and end rings to hold it together. Without the prior art rotor bars and end rings, it is necessary to develop a new method for attaching the counterweights to the rotor of a BPM motor.
The present invention provides a cost effective method for counterweight attachment, containment of the permanent magnets and clamping of the BPM rotor laminations. Upper and lower counterweights are cast from a non-magnetic material such as brass or zinc. The counterweights are cast with a thin base plate, approximately the same diameter as the laminations and with a protruding mass for balancing. Four steel inserts are cast within the counterweights to facilitate the assembly of the BPM rotor. There are two designs for the steel inserts, where one has a threaded interior hole and the other has a clearance hole. The threaded insert is used if the attachment bolt is inserted from the opposite side of the rotor and threadingly received by the insert. The clearance insert is used if the attachment bolt is inserted from the same side as the insert as detailed below. The rotor laminations contain four clearance holes for bolts and slots for the permanent magnets.
The rotor laminations, magnets and counterweights are first assembled into a sub-assembly. Two bolts are assembled through the insert having the clearance holes in the lower counterweight base plate, through the clearance holes in the laminations and then threaded into two inserts having the threaded holes in the upper counterweight. Two bolts are assembled through the insert having the clearance holes in the upper counterweight base plate, through the clearance holes in the laminations and then threaded into two inserts having the threaded holes in the lower counterweight. This sub-assembly can then be pressed onto the driveshaft of the BPM motor. While the rotor assembly has been described having two bolts being assembled in opposite directions, it is within the scope of the present invention to have all four bolts assembled in the same direction if desired. The incorporation of the steel inserts permits a metal to metal clamping for the rotor assembly with a steel connection being maintained through the assembly. This steel connection overcomes the problems associated with creeping of the prior art zinc counterweight.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.